Filtering and separation centrifuges for the aforedescribed purposes are known in a variety of configurations and, for the present purposes, only the upwardly-open basket type of filter or separation centrifuge need be considered. In such a centrifuge, a generally upwardly open perforated basket or shell is rotated at relatively high speed about an upright axis to impart centrifugal forces to the liquid ejected through the openings of the basket corresponding to several hundreds to 10.000 or more times the gravitational acceleration, i.e. from say 200 G to upwardly 10,000 G.
Reference may be made to German Pat. Nos. DT-PS 1,072,199, 1,198,295 and 1,288,990, in this connection. These patents deal with wobbling or nutating centrifuges in which the basket is constituted as a frustocone and has a wall of the centrifugation drum which is inclined ultimately at each location at such an angle to the vertical that the centrifugal force is effective to cast the material over an edge of this wall, while over the remaining portion of the drum, the inclination is such as to prevent or limit discharge over this edge.
In other words, the drum receives a wobbling or nutating movement in which the outwardly tilted edge of the drum at any instant permits the discharge of centrifuged material from the drum and induces the transport of the centrifugal material therealong.
The degree of tilt must, of course, be a function of the slip angle of the centrifuged material. The slip angle is defined as that angle with the direction of force which a surface can make in contact with a body bearing against that surface in the direction of the applied force which will enable the body or mass to slip against the retarding friction and hence slide along the surface.
In the case of the upright centrifuge, the applicable force is the centrifugal force which is effective radially and hence the wall of the drum is tilted by the wobbling or nutating action so as to decrease the angle which includes with a radius to permit slip of the centrifuge material along the wall of the drum.
The wobbling or nutating motion of the drum thus causes continuous discharge and movement without requiring an inclination of the drum wall over the entire periphery to correspond to the slip angle. In this respect, the wobbling or nutating centrifuge drum has a considerable advantage over systems in which no nutation or wobbling occurs.
Wobbling and nutating centrifuges are basically provided for the separation of relatively rapidly filtering materials. A disadvantage of such centrifuges, as constructed and designed in accordance with prior-art principles resides however in the fact that transport conditions brought about by the wobbling or nutating movement are dependent upon the sliding or slip characteristics of the centrifuged material, i.e. the coefficient .mu.. The various solids to be separated in the wobble or nutating centrifuge have different friction coefficients.
In German Pat. No. DT-PS 1,151,223, there is described a system wherein the nutating or wobble angle .tau. (i.e. the angle between the axis of the drum in its greatest-throw position included with the normal axis of vertical) is adjustable.
The apparatus, however, provided with such adjustability is extremely expensive. Furthermore, the high restoring forces which arise in the wobble or nutating centrifuge must be taken up by the bearings of tee wobble-generating head and, where some adjustability is provided, also by the adjustment mechanism. In other wobble or nutating centrifuges of the type described in the art, the angle is fixed and a variety of drums or heads with different angles, depending upon the various materials to be centrifuged must be stocked and used interchangeable in the centrifuge.